Method of drying lumber



y 1962 H. P. WOOD 3,031,767

METHOD OF DRYING LUMBER Filed Feb. 14, 1958 FIG. I

HAROLD I? wooo INVENTOR BYGZIZZfl @H/M ATTORNEYS 3,031,767 Patented May 1, 1962 fifice Filed Feb. 14, 1958, Ser. No. 715,244 3 Claims. (Cl. 34-1) This invention is for method of drying lumber. The method contemplates the use of known equipment, but the procedures are conducted in such manner that a num ber of results are achieved which have previously been considered unattainable. Foremost among the desirable results obtained is the drying of lumber in a matter of hours instead of days as required by other known processes. The lumber so dried will be ioundat the end of the drying process to be in perfect condition for immediate use without the need for subsequent conditioning. This result stems directly from the low temperatures used durmg my drying process. 7

By the herein disclosed process not only is the lumber dried in a very short time as compared with known and existing procedures, but also the cost is found to be lower than drying by conventional steps or by any other heretofore known process.

When the term lumber is used herein, it refers broadly to any type of wood which before use is customarily subjected to drying operations, whether an drying, kiln drying or a. combination of both. The method that will hereafter be described and claimed is practiced in drying batches of lumber. A typical batch of lumber considered in relation to kiln drying is 1,000 board feet more or less, and the present process is especially adapted for carrying out batch drying.

The present method contemplates the joint use of dielectric internal heating of the wood and minimizing loss of such heat by external circulation of hot air. In the dielectric heating, the frequencies found-satisfactory are preferably in the order of 3.5 to 6.5 megacycles, but it will be understood that other frequencies and wave lengths may be used if desired so long as the wood is properly internally heated. Electrical machines for producing such high frequency power or radio frequency power, as it is often called, which will cause internal heating of lumber to be dried have been known in this field for many years and are currently available. One such machine is made by the Raybond Company, in Newton, Massachusetts and as the construction, use and operation of such machines are Well understood it is believed unnecessary to explain the electrical circuits involved or other technical features relating thereto. However, it is important that the power of the radio frequency oscillator capable of producing the required oscillations be held as low as possible considering the volume of wood in the batch. The selection of a relatively low powered high frequency generator has three desirable effects in connection with the present process. First, the cost of electricity is reduced because of lower power consumption and lower demand charge; secondly, during the over-all drying period the oscillator, because of its low power, will of necessity be in operation for a longer time in maintaining the wood at the correct temperature during the drying period; and thirdly, by the use of a low power generator, temperature and moisture gradients will be created more slowly, thus avoiding the development of physical stresses and fiber tensions which are the causes of internal collapse, checking and twisting.

While the radio frequency generator is used to first raise and thereafter hold the temperature of the wood at the desired degree, it is also necesary according to the present process to keep the batch of wood in a heated atmosphere so that the relatively high cost heat'induced in the wood by the radio frequency generator will not be lost to the surrounding atmosphere and to the component parts of the dry kiln at a faster rate than the particular specie of wood'being dried will give up its moisture content. Accordingly it is proposed that heated air be circulated about the batch of wood (the batch being located in a closed room or kiln) with the temperature of the surrounding air always being slightly below that of the internal temperature of the wood. The humidity of the air is such that it will evaporate more or less of the moisture coming to the surface of the wood but will not cause evaporation at a rate faster than the rate of moisture travel from within the wood to the surface thereof as permitted by the particular specie being dried.

, Heretofore it has been thought that drying of lumber was a function of time plus temperature and therefore the use of radio frequency power required an increase (continuous or in steps) in the internal temperature of the wood with a corresponding increase in the temperature of the surrounding circulating air. The prior art on this subject indicates that it has heretofore been thought that the only way lumber drying could be speeded up was by increasing the temperature just as fast as the ability of the particular specie to withstand internal damage would permit. On the other hand, it has been the view of prior workers in this field that if the lumber were maintained at a low temperature, thus avoiding any internal damage, the drying would necessarily be slow even at the outset and as the drying progressed the rate of drying would become progressively less. For example, the prior art has shown that in a kiln maintained at 130 degrees Fahrenheit, it would take about three days to reduce the moisture content from 60% to 2.5% but an additional ten days would be required to reduce the moisture content from 25% to 6%.

By low temperature is meant, in the present case, not above 130 to 140 degrees Fahrenheit. Accordingly, a further feature of the present invention is the maintenance of a temperature in the wood during the drying period of approximately degrees which may be increased somewhat but probably should not exceed degrees. This temperature in the wood, when coupled with a surrounding air temperature of approximately 5 degrees lower, will result in drying lumber to commercial require ments in a period of 18 to 30 hours depending upon the particular type of wood involved and, of course, upon the initial percentage of moisture present therein. When the method is properly practiced, as wi l hereinafter be explained, moisture will leave the wood at the outset at a rate of about 2% per hour, the rate dropping somewhat as the end of the drying period approaches. This very high rate of drying at a low temperature is completely at variance with the teachings of the prior art in which it has been considered necessary for fast drying to progressively raise both the interior and exterior temperatures with the final temperatures being in the vicinity of the boiling point.

Because my drying procedures are conducted at a low internal wood temperature and with a somewhat lower temperature of the surrounding air, the wood in the finished dried condition is completely undamaged. No checking appears on the surface. There is no case hardening, and there is no interior damage. In other words, perfectly dried lumber comes from the kiln at the termination. of the drying period, such period being not in excess of 30 hours for the most difiicult to dry Woods and correspondingly shorter periods for easier to dry woods. The low temperature at which the wood is maintained is a temperature probably not higher than the temperature the wood in a growing tree may readily reach during the hot days of the summer. That is to say, the wood fibers during my drying process are not subjected to heating conditions any more severe than they were subjected to during their normal growth. The temperature to which I subject the wood produces moisture and temperature gradients from the interior to the suface similar to those under which the tree has evolved during the growing state. These low temperatures are believed to account in part for the undamaged condition in which the Wood as lumber goes through the drying process of the present invention.

It must be further emphasized that an important feature of the present process not taught by the prior art is the maintenance of a surrounding atmosphere that is lower than the internal wood temperature but only slightly lower. Since the external temperature is lower, the second law of thermo-dynamics comes into operation, resulting in movement of heat from the wood interior to the surface. The relatively low temperatures interiorly and exteriorly applied have the effect of causing the moisture to move from the interior to the wood surface at a surprisingly rapid rate because temperature and moisture gradients are being maintained which allow the passage of moisture as readily from within the lumber to the surface as was possible in the live tree as it grew during its life. At the same time, as previously pointed out, by having the exterior atmosphere at about degrees more or less below the internal wood temperature, heat loss from the wood to the ambient atmosphere is low. That is, the heat in the wood produced by a relatively high cost method (dielectric heating) is lost only slowly to the surrounding moving air. Thus the electric cost is held to a minimum through the use of air that is heated by conventional means at a much lower cost.

By the use of a low power high frequency generator, it has been found that a batch of 1,000 board feet of wet lumber may be brought from say 50 degrees to 120 degrees in about half an hour using a kilowatt generator. Thereafter the internal temperature of the wood may be maintained at 120 degrees through the application of radio frequency power for perhaps a total of 10 minutes per hour. A low power generator is much to be preferred over the more conventionally sized 40 kilowatt machine heretofore used with this quantity of lumber. It is thought that best results would be obtained if the generator were of such low power that is could be run continuously while maintaining the internal wood temperature at a constant low figure. However, the power requirement that would be sufiicient to maintain the moisture content of the wood at 120 degrees if the machine were running continuously would be so low that such generator probably could not be readily brought to function with a batch of Wood of the volume of 1,000 board feet. Accordingly, under the present state of the dielectric heating art there must be a compromise. The generator must be large enough to generate the high frequency currents in the volume of lumber in the batch but thereafter when the temperature has been brought up to the required low figure of say 120 degrees the generator will have to be run intermittently if the temperature is to be limited to that figure. Generally speaking, in those cases where the generator is being used with a single kiln, the generator will be controlled to resume operation after the internal wood temperature has dropped perhaps two or three degrees and to be cut off after the temperature has risen again to 120 degrees. Because of the intermittent operation, there is of necessity a small fiuctation of the internal wood temperature, but this is held to a practical minimum. Maximum electrical economy can be obtained by using a single generator with a plurality of kilns containing in total a sufficient quantity of lumber to require substantially continuous generator operation to maintain the required low temperature in all kilns of say 120 degrees Fahrenheit. By switching the generator from kiln to kiln in a properly balanced power to total lumber ratio, the generator may run substantially continuously and the lumber temperature in each kiln will be held within required narrow limits. Upon the introduction of suitable automatic controls, the temperature fluctuation might be held within very narrow limits. ideally, it is believed as indicated above that the fastest drying will take place if the radio frequency power is applied continuously, but only in such strength that the internal wood temperature will not go above the desired low figure. It is thought that the molecular oscillations of the moisture molecules are largely responsible for the increase in the capillary action and osmosis which occurs during the drying process and resulting in the unusually rapid movement of moisture to the wood exterior. By my process, I prevent uneven shrinkage and closing of the air pores, particularly those near the surface of the lumber. These obtainable results clearly distinguish my process from all other known drying processes in which the outside of the lumber is dried faster than the inside.

In the teachings of the prior art, the temperatures and humidities have always been considered as critical for good drying, but in my present process the important features are the relatively small temperature differential between the internal wood temperature and the surrounding atmosphere which for best results should be limited to not less than 3 nor more than 7 degrees, but as mentioned before, the particular humidity of the circulating air Is not critical at all. The surrounding air temperature must always be below the wood temperature. If the wood temperature drops below the air temperature, this will temporarily slow the drying process, as the outside would then dry faster than the inside. Uneven shrinkage would start to take place, and there would be rapid closing of the surface pores of the wood with the eventual near stopping of uniform moisture travel from within to the surface resulting in the fiber stresses and strains that other drying methods produce. When this situation develops, the only free passage of moisture from the interior to the outside air is via the length of the board through the ends. This is the cause of collapse and other forms of degrade that go hand in hand with either natural or conventional kiln drying.

During those periods in the drying process when the radio frequency power is off and the internal wood temperature is falling slowly and with the temperature of surrounding atmosphere always somewhat lower, it has been found that drying continues effectively over a limited temperature drop. Before the rate of drying appreciably diminishes, however, the radio frequency power is again applied. It takes time for the 120 degree internal temperature to work outwardly through the wood to the surface. Thus even withthe generator off, the moisture gradient is maintained and drying continues at the same rate.

Following the method of the present invention, a batch of oak (1,000 board feet) has been dried at a total cost for electricity of $5.94 (2 cents per kilowatt hour rate). The cost of heating the small quantity of recirculated surrounding air for the 20 to 30 hours required on one to three inch lumber is negligible, perhaps less than cents. These figures, combined with the decreased inventory required and the short use of the kiln per batch, results in a total drying charge per 1,000 board feet substantially below current costs according to conventional practices and even farther below costs in processes in which radio frequency power has been used according to prior art teachings, as far as I ant aware. With two kilns operated from one generator and at 1 cent per kilowatt hour current, the cost would drop to $2.97 per 1,000 feet. Many placesin the United States I have power available at .7 cent per kilowatt hour. Thus With the foregoing in mind, reference will now be made to apparatus by which the process of the invention may be carried out.

Referring to the drawings,

FIG. 1 is a vertical cross-sectional view of a kiln and related equipment designed to carry out the method.

FIG. 2 is a schematic showing to indicate a high or radio frequency generator and the attached electrodes for applying the power of the generator to the lumber in the kiln whereby dielectric heating of the lumber will occur.

Referring to FIG. 1, the lumber may be positioned in conventional manner in a closed room or kiln. In the present instance, the boards B have been piled in spaced apart relation (separated by slat S) on a truck which may be rolled into and out of the kiln. The kiln walls and top may be in the form of a one-piece hood H, the walls being indicated at 2 and the top at 4. This hood is an integral structure having electrical shielding 6 throughout so as to protect the adjacent areas from electrical interference that might otherwise be produced through the use of the generator. The hood H may be raised vertically by means of lifting rings 8 and 10 shown broken away in part. The lower circumferential edge of the kiln may rest on a rectangular base 12, having an inner flange 14 adapted to make a suitably sealed joint. The floor 16 may have suitably located passages 18 and 20 therethrough through which heated air entering through conduit 22 may circulate upward, about and between the spaced boards and thence flow outwardly through another conduit 24 to a condenser if desired, after which it may be recirculated back into the kiln through conduit 22. The heating and circulation of air in kilns is well understood, and any convenient mechanism may be used for this purpose. The form herein shown is merely suggestive and the particular construction has no bearing on the invention.

A baffle 26 may be provided, running lengthwise of the kiln and acting to insure better circulation of the heated air through and between the pieces of lumber.

A high frequency generator 28 is indicated in FIG. 2, this generator being capable of producing high frequency alternating current through the lumber B positioned be tween the electrodes 30 and 32 for the purpose of cansing dielectric heating of the lumber.

As shown in FIG. 1, the lead in conductors 34 and 36 connected to electrodes 32 and 30 respectively come from the generator which will be conveniently located exterior of the kiln. The two electrodes are placed vertically on opposite sides of the pile of lumber and will be generally in contact therewith, although due to irregularities normally present in piling the lumber the electrodes will not be in contact with all of the boards and it will be understood that such contact is not necessary. The generator should preferably be of relatively low power so that while holding the range of temperature to a few degrees with the maximum at about 120 degrees Fahrenheit and the minimum preferably not less than 10 degrees lower, the generator will be in operation for the longest possible time. A 10 kilowatt generator has been found satisfactory for use in drying a batch of 1,000 board feet but such generator must be operated intermittently. Continuous operation would cause the temperature to rise above the predetermined maximum. As previously mentioned, more or less continuous operation of the generator can be obtained by switching the power back and forth between two or more kilns. v

If a generator of still lower power can be made to function with the pile of lumber so that the on time may be increased without raising the wood temperature above the predetermined maximum degree, it is believed that the drying process will be improved still further. Therefore it is considered that either intermittent or confinuous operation of the generator falls within the scope of the present invention so long as the wood temperature is maintained within the small limits indicated and held below a selected predetermined relatively low temperature in the order of 120 to 130 degrees Fahrenheit, and the surrounding air temperature is always maintained at a temperature a few degrees below the current wood temperature.

With the lumber in position within the kiln, generator 28 may be put in operation and within perhaps half an hour more or less the internal temperature of the boards in the batch will be brought to the desired low temperature which for purposes of illustration will be 120 degrees. In the meanwhile the hot air entering through conduit 22 will be brought up to a temperature of degrees. As soon as these temperatures are reached, the generator and steam heating the kiln air are temporarily turned off, but circulation of the air is continued. The drying process is now underway. Moisture in the lumber begins to move continuously from the warmer interior to the slightly cooler exterior and evaporation commences. With the generator off, the internal temperature of the lumber begins to fall and the air temperature falls correspondingly at a faster rate. Thermometers in the form of thermocouples, as at 38 and 40, with indicating devices at 42 and 44 respectively, may be strategically located throughout the pile of lumber to advise as to temperature conditions in the wood so that the generator 28 may be turned on and off periodically to maintain the internal wood temperature close to degrees. Likewise controls on the steam supply will maintain the circulating air at a temperature about 5 degrees less than the internal wood temperature.

Representative samples of the wood being dried may be removed from the kiln every hour or two to be tested so that the progress of the drying may be known. Drying continues in this easily carried out manner for a period of 20 to 30 hours depending upon the wood and the initial moisture content, at the end of which time it will be found that the wood will have been reduced in moisture content to ordinary commercial requirements, namely 6 to 10 percent. The process may then be stopped, and upon removal of the batch of lumber from the kiln it will be found that the lumber is ready for immediate use without the necessity of subsequent 'so-called conditioning.

It is my intention to cover all changes and modifications of the example of the invention herein chosen for purposes of the disclosure which do not constitute departures from the spirit and scope of the invention.

I claim:

1. The method of drying lumber comprising the steps of placing a quantity of spaced pieces of lumber to be dried in an enclosed space, raising the internal temperature of the pieces of lumber to a low drying temperature not greater than about F. solely by the application of a high frequency energy field to the lumber, heating air to a predetermined temperature approximately 5 F. below said low drying temperature, circulating the heated air around the lumber in said enclosure, and maintaining the internal temperature of the lumber at said low drying temperature and the circulating air at said predetermined temperature to establish and maintain a continuous temperature gradient for producing heat flow from within the individual pieces of said lumber outwardly to said circulating air until said lumber is dried to less than 10% moisture content without being heated above said low drying temperature.

2. The method of claim 1 in which said energy field is a radio frequency electromagnetic field.

3. The method of drying lumber comprising the steps of dielectrically heating a quantity of lumber to raise the internal temperature of each piece of lumber not to exceed approximately 130 F., circulating air approximately 5 cooler than said internal temperature around said pieces, maintaining the temperature conditions substantially constant within the lumber and in the surrounding air to produce a continuous temperature gradient and resuiting vapor pressure gradient in said pieces which results References Cited in the file of this patent in heat flow and moisture flow outwardly from the in- UNITED STATES PATENTS terior of each piece of lumber and continuing said temperature conditions to produce said gradients until said g Mann et lumber is dried to less than 10% moisture content. 5 43,618 Wood e 

